10 



MICROTIN^ 



Fig. 3. — Arvicola ampldhius Linnseus. 



Ventral views of skulls in different stages of growth. Stages I, II, 



and III (same specimens as shown in Fig. 1). 



In Stage I the rostrum is short and broad, the molars are relatively 



large, the palate is simple and level throughout, the pterygoid fossae are 



short, and the bullae are rounded. In the later stages the rostrum becomes 



progressively longer and narrower as the incisors increase in length, the 



